In this context emergency brakes which incorporate a friction braking element associated with a rotating member of the transmission are known. Typically, although not exclusively, such braking elements comprise a pack of alternating disks and counter-disks in which the disks are fixed to the rotating member while the counter-disks are fixed to a stationary member.
Provision is made for the use of a resilient braking device to apply a thrust load on the pack of disks and counter-disks so as to compress them in order to transmit a braking torque to the rotating member. This braking device can be subjected to the action of a hydraulic system with an opposing load which is sufficient to cancel out the action of the thrust load, enabling the disks and counter-disks to move apart and counteract the braking torque. When the hydraulic system is active, relative rotation between the disks and counter-disks is free.
In practice, when the engine is not running the brake is activated by the force exerted on the braking pack of disks and counter-disks by one or more springs. When the engine is running this force is counteracted by the thrust of a hydraulic piston acting on those springs. The brake comes into action when a fault in the vehicle causes the engine to stop and as a consequence pressure is lost in the hydraulic system and the thrust of the hydraulic piston is reduced to nothing.
The main disadvantage of such devices lies in the fact that when the abovementioned emergency brake is activated the kinetic energy of the vehicle is dissipated as heat in the braking pack, in a time of typically between 2 and 5 seconds. Again, for safety reasons, it is typically required that the brake should be able to act several times consecutively without suffering damage or significant loss of performance.